Growth and Development of Horticultural Crops (1+1)

Brassinosteroids

• Brassinosteroids (BRs) are a class of polyhydroxysteroids that have been recognized as a sixth class of plant hormones. These were first explored nearly forty years ago when Mitchell et al. reported promotion in stem elongation and cell division by the treatment of organic extracts of rapeseed (Brassica napus) pollen Brassinolide was the first isolated brassinosteroid in 1979 when it was shown that pollen from Brassica napus could promote stem elongation and cell divisions, and the biologically active molecule was isolated. The yield of brassinosteriods from 230 kg of Brassica napus pollen was only 10 mg. Since their discovery, over 70 BR compounds have been isolated from plants.

Brassinolide

• The BR is biosynthesized from campesterol. The biosynthetic pathway was elucidated by Japanese researchers and later shown to be correct through the analysis of BR biosynthesis mutants in Arabidopsis thaliana, tomatoes, and peas. The sites for BR synthesis in plants have not been experimentally demonstrated. One well-supported hypothesis is that all tissues produce BRs, since BR biosynthetic and signal transduction genes are expressed in a wide range of plant organs, and short distance activity of the hormones also supports this. Experiments have shown that long distance transport is possible and that flow is in an acropetal direction, but it is not known if this movement is biologically relevant. Brassinosteroids are recognized at the cell membrane, although they are membrane-soluble.
• BRs have been shown to be involved in numerous plant processes:
• Promotion of cell expansion and cell elongation; works with auxin to do so.
• It has an unclear role in cell division and cell wall regeneration.
• Promotion of vascular differentiation; BR signal transduction has been studied during vascular differentiation.
• Is necessary for pollen elongation for pollen tube formation.
• Acceleration of senescence in dying tissue cultured cells; delayed senescence in BR mutants supports that this action may be biologically relevant.
• Can provide some protection to plants during chilling and drought stress.
• Extract from the plant Lychnis viscaria contains a relatively high amount of Brassinosteroids. Lychnis viscaria is said to increase the disease resistance of surrounding plants. In Germany, extract from the plant is allowed for use as a "plant strengthening substance."
• 24-Epibrassinolide (EBL), a brassinosteroid isolated from Aegle marmelos Correa (Rutaceae), was further evaluated for the antigenotoxicity against maleic hydrazide (MH)-induced genotoxicity in Allium cepa chromosomal aberration assay. It was shown that the percentage of chromosomal aberrations induced by maleic hydrazide (0.01%) declined significantly with 24-epibrassinolide treatment.
• BRs have been reported to counteract both abiotic and biotic stress in plants. Application of brassinosteroids to cucumbers was demonstrated to increase the metabolism and removal of pesticides, which could be beneficial for reducing the human ingestion of residual pesticides from non-organically grown vegetables.
Table 1. Physiological effects of brassinosteroids in plants


Cell level	Whole plant level
Stimulation of elongation and fission	Growth promotion
Effect on hormonal balance	Increase in the success of fertilization
Effect on enzyme activity; H.-pump activation	Shortening the period of vegetative growth
Activation of protein and nucleic acid synthesis	Size and quantity of fruits increase
Effect on the protein spectrum and on the amino acid composition of proteins	Effect on the content of nutritive components and fruit quality improvement
Effect on the fatty acid composition and on the properties of membrane	Increased resistance to unfavourable environmental factors, stress and diseases
Enhancement of the photosynthetic capacity and of translocation of products	Crop yield increase


VLADIMIR Khripach et al. (2000)